docs: fix memory leak of appsink example in manual
docs: fix memory leak of appsink example in manual

https://bugzilla.gnome.org/show_bug.cgi?id=721076

cablelabs/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,ensonic/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,StreamUtils/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,mparis/gstreamer,centricular/gstreamer,centricular/gstreamer,ensonic/gstreamer,StreamUtils/gstreamer,ensonic/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,krichter722/gstreamer,surround-io/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,cablelabs/gstreamer,jpakkane/gstreamer,ensonic/gstreamer,cfoch/gstreamer,Lachann/gstreamer,surround-io/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,surround-io/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,cablelabs/gstreamer,jpakkane/gstreamer,Lachann/gstreamer,Lachann/gstreamer,jpxiong/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,cfoch/gstreamer,centricular/gstreamer,mparis/gstreamer,jpakkane/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,surround-io/gstreamer,surround-io/gstreamer,centricular/gstreamer,jpakkane/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,mparis/gstreamer